This invention relates to die attach adhesives and encapsulants containing silane and cross-linking functionalities for use in semiconductor packages, and to semiconductor packages in which the semiconductor is adhered to a substrate with the die attach adhesive.
Adhesive compositions are used in many industries, such as the microelectronics industry, where good adhesion to metal and organic substrates and low viscosity for easy dispensability are important requisites.
Adhesion to metal and organic substrates is not always easily achievable, and the addition of silane adhesion promoters to adhesive formulations is one means of correcting this deficiency. Commonly used and commercially available silanes are small molecules that tend to volatilize significantly before the cure temperature of the adhesive is reached. Because silanes tend to be subject to hydrolysis, the addition of higher amounts of the silanes to offset the volatility could lead in turn to the presence of moisture in the adhesive compositions. This could be a problem in many applications. With reference to microelectronic devices, moisture creates the potential, through corrosion of circuits or voiding of the moisture and delamination of device packaging, for eventual failure of the device.
As a solution to these problems this specification discloses adhesion promoting resins with sufficient molecular weight to give lower volatility than the silanes currently commercially available. The adhesion promoting resins disclosed in this specification are curable compositions that are suitable for use as adhesives, encapsulants, or sealants, particularly for applications within the microelectronic industry.
This invention is a semiconductor package using a curable die attach adhesive and encapsulant formulation comprising a resin that contains an oligomeric or polymeric segment and a silane segment, a conductive or nonconductive filler, and optionally, an initiator. The silane segment on the resin will be present at the terminus of an oligomeric segment, or at a terminus or the termini of a polymeric segment, or at the terminus or termini and pendant from a polymeric segment.
The oligomeric or polymeric segment of the resin used in the adhesive and encapsulant formulation will contain at least one carbon to carbon double bond, and can be provided by a precursor resin or compound that is commercially available. Alternatively, the precursor resin or compound can be synthesized by the practitioner.
Suitable oligomeric segment precursors are linear or branched hydrocarbons having 1 to 50 carbon atoms, which contain a reactive functionality, excluding epoxy, for ultimate reaction with a co-reactive functionality, excluding epoxy, on the silane segment precursor. Exemplary precursors include N-methylallylamine, N-ethyl-2-methylallyl-amine, diallylamine, N,Nxe2x80x2-diethyl-2-butene-1,4-diamine, N-allylcyclopentyl-amine, allylcyclohexylamine, 2-(1-cyclohexenyl)ethylamine, and other compounds such as the alcohols containing unsaturation disclosed later in the Examples in this specification.
Exemplary suitable polymeric segments are homopolymers of 1,3-butadiene, or copolymers of ethylene, propylene and a diene. Poly(butadienes) are commercially available with hydroxyl-, amino-, halo-, and isocyanate-functionality.
The unsaturated oligomeric or polymeric segment may be any molecular weight and structure desired by the practitioner, provided it has at feast one carbon to carbon double bond for subsequent cross-linking. The oligomeric or polymeric segment may contain heteroatoms, such as, silicon, sulfur, nitrogen, or oxygen; may contain functional groups, such as hydroxyl, urea, carbamate, or ester; may contain cyclic or aromatic moieties. The at least one carbon to carbon double bond can be located within the oligomeric or polymeric backbone or chain, or pendant from the chain.
The silane segment will have the structure 
in which n is simultaneously for each position 0, 1, or 2; R1 is a methyl or ethyl group; R2 is a vinyl group, an aromatic group, or a linear or branched alkyl group, preferably of 1 to 4 carbon atoms, and more preferably methyl or ethyl; A is a linear or branched alkyl group or a cyclic or aromatic group, and L is a linking group resulting from the reaction of a functional group on the silane precursor and a functional group on the oligomeric or polymeric precursor, with the proviso that neither functional group is an epoxy. There can be more than one silane segment per molecule of resin.
Examples of commercially available silanes suitable as precursors for the silane segment are gamma-isocyanatopropyltriethoxysilane, gamma-aminopropyltriethoxy-silane, gamma-aminopropyltrimethoxysilane, N-beta-(aminoethyl)-gamma-aminopropyltrimethoxysilane, triaminofunctional silane, bis-(gamma-trimethoxysilylpropyl)amine, N-phenyl-gamma-amino-propyltrimethoxysilane, N-beta-(aminoethyl)-gamma-aminopropylmethyldimethoxysilane, gamma-mercaptopropyl-trimethoxysilane, 3-aminopropyldimethylethoxysilane; 3-bromopropyltrimethoxysilane, 3-chloropropylmethyldimethoxysilane, 3-chloropropyltrimethoxysilane, dimethylmethoxychlorosilane, methyldimethoxysilane, and methyldiethoxysilane.
The resin is synthesized through a reaction between the precursor (also, hereinafer, starting material) for the unsaturated segment and the precursor for the silane segment. Although any appropriate synthetic route convenient to the practitioner may be used, suitable synthetic routes are through condensation or addition reactions between co-reactive functionalities, excluding epoxy, on the precursors. Examples of these reactions are illustrated later in this specification.
In one embodiment of the resin, the silane segment will be located at the termini of the oligomeric or polymeric segment and the at least one carbon to carbon double bond will be located within the oligomeric or polymeric chain or pendant from the chain. In this embodiment, the resin will have the structure 
in which Q is an oligomeric or polymeric group containing at least one carbon to carbon double bond, derived from an oligomeric or polymeric precursor containing carbon to carbon unsaturation; n is simultaneously for each position 0, 1, or 2; R1 is a methyl or ethyl group; R2 is a vinyl group, an aromatic group, or a linear or branched alkyl group; A is a hydrocarbyl group, for example, a linear or branched alkyl group or a cyclic alkyl or alkenyl group, or an aromatic group; L is a linking group resulting from the reaction of a functional group, excluding epoxy, on the precursor for the segment containing silane and a functional group, excluding epoxy, on the precursor for the segment containing the at least one carbon to carbon double bond.
In another embodiment, the silane segment will be located at the termini of a polymeric chain and pendant from the polymeric chain. The polymeric segment will contain at least one carbon to carbon double bond located within the polymeric chain. The degree of substitution of silane onto pendant functionality can be controlled by reaction stoichiometry as desired by the practitioner. In this embodiment the adhesion promoting resin will have the structure: 
in which Q, A, L, R1, and R2 are as described before.
The functional groups on the oligomeric or polymeric precursors through which the silane is to be linked include hydroxyl, halide, amine, isocyanate, carboxylic acid, acid chloride, and vinyl double bonds. The functional groups on the silane precursor, for reaction with the functional groups on the oligomeric or polymeric precursor, include hydroxyl, amine, mercapto, isocyanate, halide, and a hydrosilation reactive hydrogen on a silicon atom. Consequently, it will be understood that the linking group can be a direct bond or an alkyl group, or can have a structure, such as 
in which R4 is hydrogen, an aromatic, or an alkyl group of 1 to 6 carbon atoms, and preferably is hydrogen, methyl or ethyl.
Within adhesive and encapsulant formulations, these resins will be used in an effective amount to cause adhesion promotion. In general, an adhesion promoting amount will range from 0.005 to 20.0 percent by weight of the total formulation. For some resins, particularly those in which an effective amount of cross-linking for the specific end-use can be achieved through the unsaturation in the polymeric or oligomeric chain, the amount used in the formulation will comprise the total or a major portion of resin used. In that case, the resin will be present in an amount up to 99% by weight.
In those formulations in which the resin is used in an effective amount to cause adhesion, the formulation may also contain at least one additional curable resin known for use as an adhesive, coating, encapsulant or sealant. Examples of such additional resins include epoxies, electron donor resins (for example, vinyl ethers, and resins that contain carbon to carbon double bonds attached to an aromatic ring and conjugated with the unsaturation in the aromatic ring, such as compounds derived from cinnamyl and styrenic starting compounds), and, electron acceptor resins (for example, fumarates, maleates, acrylates, and maleimides).
Preferably, the formulation will contain a curing agent, such as, a thermal initiator or photoinitiator, or both, present in an effective amount to cure the formulation. In general, those amounts will range from 0.5% to 30%, preferably 1% to 20%, by weight of the total organic material (that is, excluding any inorganic fillers) in the formulation. Preferred thermal initiators include peroxides, such as butyl peroctoates and dicumyl peroxide, and azo compounds, such as 2,2xe2x80x2-azobis(2-methyl-propanenitrile) and 2,2xe2x80x2-azobis(2-methyl-butanenitrile). A preferred series of photoinitiators is one sold under the trademark Irgacure by Ciba Specialty Chemicals. In some formulations, both thermal initiation and photoinitiation may be desirable: the curing process can be started either by irradiation, followed by heat, or can be started by heat, followed by irradiation. In general, the formulations will cure within a temperature range of 70xc2x0 C. to 250xc2x0 C., and curing will be effected within a range of ten seconds to three hours. The actual cure profile will vary with the components and can be determined without undue experimentation by the practitioner.
The formulations may also comprise conductive or nonconductive fillers. Suitable conductive fillers are carbon black, graphite, gold, silver, copper, copper alloys, platinum, palladium, nickel, aluminum, silver coated copper, silver alloys with platinum or palladium, bronze or brass alloys, silicon carbide, boron nitride, diamond, and alumina. Suitable nonconductive fillers are particles of vermiculite, mica, wollastonite, calcium carbonate, titania, sand, glass, fused silica, fumed silica, barium sulfate, and halogenated ethylene polymers, such as tetrafluoroethylene, trifluoroethylene, vinylidene fluoride, vinyl fluoride, vinylidene chloride, and vinyl chloride. If present, fillers generally will be in amounts of 20% to 90% by weight of the formulation.
In a further embodiment of this invention, the above described curable compounds are formulated into a die attach adhesive and used to adhere a semiconductor silicon chip to substrate, which assembly of chip and substrate is then encapsulated to give a semiconductor package. Various processes are known in the art for assembling and packaging an electronic device.
One such process comprises a first step of mechanically adhering the semiconductor chip to a central mounting pad in a metal lead frame with the electrically active surface of the chip exposed away from the mounting pad. A die attach adhesive, such as described earlier in this specification, is used for this purpose. The adhesive is applied to either the mounting pad or the passive face of the semiconductor chip, and the chip then contacted with heat and/or pressure to the mounting pad.
The leads of the lead frame are then bonded electrically to the contact pads located on the perimeter of the semiconductor chip by soldering fine gold wires to the contact pads and leads to effect an electrical interconnection. The chip and the lead frame are then encapsulated in a plastic package by an injection or transfer molding process, after which the electrical leads are trimmed to the desired length and formed to the desired configuration to provide a second level interconnect to a printed circuit board or other type of substrate.
In the following Synthetic Procedures and Examples, the reaction stoichiometry for the syntheses of starting materials and resins is given in molar equivalents per molecule of reactant. Some of the reactants have more than one functionality per molecule, and the ratio of molar equivalent per functionality can be varied to give a predetermined degree of reactivity or substitution. For convenience, the depicted structures show full substitution along the polymeric chain. For example, the level of hydroxyl functionality added to a poly(butadiene) through the reaction of mercaptoethanol with the pendant vinyl functionality on the precursor poly(butadiene) can be varied by the molar amounts of mercaptoethanol reacted, and can range from one molar equivalent to full substitution of the 20% of the pendant vinyl groups along the poly(butadiene) polymer chain. Similarly, the level of silane functionality can be varied by adjusting the mole equivalents of functionalities to be reacted.
PROCEDURE 1: Reaction of mercaptoethanol with pendant vinyl functionality on poly(butadiene) (PBD) The synthetic procedure is conducted according Boutevin, G., Ameduri, B., Boutevin, B., Joubert, J-P., Joumal of Applied Polymer Science, 75, 1655-1666, 2000, in a ratio of 1:1 mole equivalent of mercapto functionality to vinyl functionality, at the level of substitution of desired. PBD and 2-mercaptoethanol are degassed under nitrogen for ten minutes. When the temperature of the solution reaches 85xc2x0 C., 1.6 mole equivalent of azobisisobutyronitrile (AIBN) per mole equivalent of mercapto-ethanol is added, followed by a second similar addition after four hours. When the reaction time is eight hours, the residual mercaptoethanol is titrated and sufficient additional AIBN added to force the reaction to completion.
PROCEDURE 2: Reaction of isocyanate with alcohol. One mole equivalent of isocyanate is solvated in toluene in a three-necked flask equipped with a mechanical stirrer, addition funnel and nitrogen inlet/outlet. The reaction is placed under nitrogen, and a catalytic amount of dibutyltin dilaurate (catalyst) is added with stirring as the solution is heated to 60xc2x0 C. The addition funnel is charged with one mole equivalent of alcohol dissolved in toluene. This solution is then added to the isocyanate solution over 10 minutes, and the resulting mixture is heated for an additional 3 hours at 60xc2x0 C. After the reaction is allowed to cool to room temperature, the solvent is removed in vacuo to give the product.
PROCEDURE 3: Reaction of gamma-mercaptopropyltrimethoxysilane SM-11 with pendant vinyl functionality on poly(butadiene). The synthetic procedure is conducted according to Schapman, F., Couvercelle, J. P., and Bunel, C, Polymer, 1998, Vol. 39, No. 20, in a ratio of 1:1 mole equivalent of gamma-mercaptopropyltrimethoxysilane to vinyl functionality, at the level of substitution desired. The level of silane functionality added can be varied by the molar amounts of mercaptosilane reacted and can range from one molar equivalent to full substitution of the 20% of the pendant vinyl groups along the poly(butadiene) polymer chain. Poly(butadiene) and gamma-mercaptopropyltrimethoxysilane are degassed under nitrogen for ten minutes. When the temperature of the solution reaches 85xc2x0 C., 1.6 mole equivalent AIBN is added, followed by a second similar addition after four hours. When the reaction time is eight hours, the residual mercaptosilane is titrated and sufficient AIBN added to force the reaction to completion. For illustration purposes only, full substitution of the pendant vinyl groups is shown in the structures in the Starting Materials and in the Examples.
PROCEDURE 4: Reaction of isocyanate with amine. One mole equivalent of isocyanate is solvated in toluene in a three-necked flask equipped with a mechanical stirrer, addition funnel and nitrogen inlet/outlet. The reaction is placed under nitrogen and the solution heated to 60xc2x0 C. The addition funnel is charged with one mole equivalent of amine in toluene. This solution is added to the isocyanate solution over 10 minutes, and the resulting mixture heated for an additional 3 hours at 60xc2x0 C. After the reaction is allowed to cool to room temperature, the solvent is removed in vacuo to give the product.
PROCEDURE 5: Reaction of alkyl halide with amine. One mole equivalent of alkyl halide is solvated in THF in a three neck flask equipped with a mechanical stirrer and addition funnel. The addition funnel is charged with one mole equivalent of amine in THF. This solution is added to the alkyl halide solution over 10 minutes at 0xc2x0 C., and the resulting mixture is stirred for 12 hours at room temperature. The solvent is removed in vacuo and to the resulting mixture is added ether and water. The organic layer is extracted and dried over MgSO4. The solvent removed in vacuo to give the product.
PROCEDURE 6: Reaction of alkyl halide with alcohol. One mole equivalent of alcohol, excess amount of 50% NaOH, a catalytic amount of tetrabutyl ammonium hydrogen sulfate, and one mole equivalent of alkyl halide in toluene are stirred for 5 hours at 53xc2x0 C., then for 5 hours at 75xc2x0 C. The reaction is allowed to cool to room temperature and the organic layer extracted and washed with brine three times. The isolated organic layer is then dried over MgSO4, filtered and the solvent removed in vacuo to give the product.
PROCEDURE 7: Conversion of alcohol functionality to chloride functionality. The synthetic procedure is conducted according to E. W. Collington and A. I. Meyers, J. Org. Chem. 36, 3044 (1971). A stirred mixture of one mole equivalent of alcohol and 1.1 mole equivalent of s-collidine under nitrogen is treated with one mole equivalent of lithium chloride dissolved in a minimum amount of dry dimethylformamide. On cooling to 0xc2x0 C., a suspension is formed which is treated dropwise with 11 mole equivalent of methanesulfonyl chloride. Stirring is continued at 0xc2x0 C. for 1-1.5 hour, when the pale yellow reaction mixture is poured over ice-water. The aqueous layer is extracted with cold ether-pentane (1:1) and the combined extracts are washed successively with saturated copper nitrate solution. This is continued until no further intensification of the blue copper solution occurs, indicating complete removal of s-collidine. The organic extracts are dried (Na2SO4) and concentrated at room temperature, providing a residue of the product.
PROCEDURE 8: Reaction of amine with acid chloride. One mole equivalent of amine and one mole equivalent of triethylamine are mixed in dry methylene chloride at 0xc2x0 C. One mole equivalent of acid chloride dissolved in dry methylene chloride is carefully added. The mixture is allowed to react for four hours. The solvent is evaporated and the crude product is purified by column chromatography using a gradient of hexane/ethyl acetate to give the product.
PROCEDURE 9: Reaction of alcohol with acid chloride. One mole equivalent of alcohol and triethylamine are mixed in dry methylene chloride at 0xc2x0 C. One mole equivalent acid chloride dissolved in dry methylene chloride is carefully added. The mixture is allowed to react for four hours. The solvent is evaporated and the crude product is purified by column chromatography using a gradient of hexane/ethyl acetate to give the product.
PROCEDURE 10: Reaction of alcohol with carboxylic acid. Into a four necked flask is charged one mole equivalent of carboxylic acid solvated in toluene, to which is added one mole equivalent of alcohol, and a catalytic amount of sulfuric acid. The flask is fitted with a Dean Stark apparatus, mercury thermometer, mechanical stirrer, and the organic solution covered with nitrogen. The reaction mixture is raised to reflux (110xc2x0 C. Reflux is maintained for approximately 4 hours or until theoretical water is obtained in the Dean Stark trap. The condensate trap is emptied to remove the collected water, and allowed to refill with fresh distillate. An equal amount of virgin solvent is charged to the flask to maintain a consistent solvent level. Following another 30 minutes of reflux, the trap is again emptied and allowed to refill; again back charging fresh solvent to replace the distillate that is removed. This process is repeated four more times in an effort to drive maximum water removal from the system. Following the final 30 minutes of reflux, the oil bath is removed and the reaction shut down. The solvent is evaporated and the crude product is purified by column chromatography using a gradient of hexane/ethyl acetate to give the product.
PROCEDURE 11: Reaction of alcohol with vinyl silane. One mole equivalent of alcohol and triethylamine are mixed in dry toluene at 0xc2x0 C. One mole equivalent of vinyl silane dissolved in toluene is carefully added. The mixture is allowed to react for four hours at room temperature. The solvent is evaporated to give the product.
PROCEDURE 12: Reaction of isocyanate with mercaptan. One mole equivalent of isocyanate is solvated in toluene in a three-necked flask equipped with a mechanical stirrer, addition funnel and nitrogen inlet/outlet. The reaction is placed under nitrogen and the solution heated to 60xc2x0 C. The addition funnel is charged with one mole equivalent of mercaptan in toluene. This solution is then added to the isocyanate solution over 10 minutes, and the resulting mixture is heated for an additional 3 hours at 60xc2x0 C. After the reaction is allowed to cool to room temperature, the solvent is removed in vacuo to give the product.
PROCEDURE 13: Reaction of isothiocyanate with alcohol. One mole equivalent of isothiocyanate is solvated in toluene in a three-necked flask equipped with a mechanical stirrer, addition funnel and nitrogen inlet/outlet. The reaction is placed under nitrogen, and a catalytic amount of dibutyltin dilaurate (catalyst) is added with stirring as the solution is heated to 60xc2x0 C. The addition funnel is charged with one mole equivalent of alcohol dissolved in toluene. This solution is added to the isothiocyanate solution over 10 minutes, and the resulting mixture is heated for an additional 3 hours at 60xc2x0 C. After the reaction is allowed to cool to room temperature, the solvent is removed in vacuo to give the product.
PROCEDURE 14: Hydrosilation. A solution of one mole equivalent of alkene and toluene is prepared with stirring, to which is added a catalytic amount of hydrogen hexachloroplatinate(IV) hydrate (H2PtCl6, available from Aldrich). The resulting solution is heated to 80xc2x0 C. and one mole equivalent of silicon hydride is added gradually via a syringe. The resulting mixture is heated for an additional hour at 80xc2x0 C. After the reaction is allowed to cool to room temperature, the solvent is removed in vacuo to give the product.
PROCEDURE 15: Reaction of carboxylic acid with isocyanate. The synthetic procedure is conducted according to T. Nishikubo, E. Takehara, and A. Kameyama, Polymer Journal, 25, 421 (1993). A stirred mixture of one mole equivalent of isocyanate and one mole equivalent of carboxylic acid is solvated in toluene in a three-necked flask equipped with a mechanical stirrer and nitrogen inlet/outlet. The mixture is heated for two hours at 80xc2x0 C. After the reaction is allowed to cool to room temperature, the solvent is removed in vacuo to give the product.
PROCEDURE 16: Reaction of disiloxane with vinyl epoxy. A round-bottomed flask is charged with one mole equivalent of disiloxane and one mole equivalent of vinyl epoxy resin. The reaction flask is equipped with a magnetic stirrer and a reflux condenser. To this mixture is added a catalytic amount of tris(triphenylphosphine)rhodium(I) chloride, and the reaction mixture is heated to 80-85xc2x0 C. for six hours. The reaction is followed using gas chromatography by monitoring the disappearance of the starting materials and the appearance of the products. After the completion of the reaction, pure product is obtained by fractional vacuum distillation.
PROCEDURE 17: Synthesis of epoxy functional poly(butadiene). One mole equivalent of PBD is dissolved in toluene and placed in a two-necked round bottomed flask. One mole equivalent of epoxy siloxane adduct is added to the flask, and the reaction mixture is heated to 60xc2x0 C. One drop of Karstedt""s catalyst is added , and the hydrosilabon reaction is obtained and monitored by following the disappearance of Sixe2x80x94H band at 2117 cm1 in the infrared spectrum. The reaction is completed in approximately two to three hours. After cooling, the reaction mixture is poured with stirring into methanol to precipitate the grafted PBD polymer. The precipitated PBD is washed with methanol and dried in vacuo at 60xc2x0 C. for eight hours.